Wood treatment composition

A method of preserving wood which comprises impregnating a wood substrate with a wood preservative formulation comprising an aqueous ammoniacal solvent comprising ammonium hydroxide having a wood treating composition dissolved therein;

• where said composition contains a quaternary ammonium compound and a metallic cation consisting essentially of at least one of copper and zinc, where said quaternary ammonium compound can be represented by the general formula (R.sub.1 R.sub.2 R.sub.3 R.sub.4 N.sup.+)X.sup.-, where R.sub.1 and R.sub.2 are independently chosen from the group consisting of alkyl groups having 1 to 3 carbon atoms, R.sub.3 is chosen from the group consisting of alkyl groups having 8 to 20 carbon atoms, and R.sub.4 is chosen from the group consisting of alkyl groups having 8 to 20 carbon atoms, aryl groups and aryl-substituted alkyl groups where said substituted alkyl groups have 1-3 carbon atoms and X.sup.-- is chosen so as to render said quaternary ammonium compound soluble in said ammoniacal solvent where said quaternary ammonium compound and said metallic cation are present in said formulation in biocidally effective amounts, and where the ratio of said quaternary ammonium compound to said metallic cation ranges from about 0.003 to about 30.